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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="hash.combine"></a><a class="link" href="combine.html" title="Combining hash values">Combining hash values</a>
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<p>
      Say you have a point class, representing a two dimensional location:
    </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">point</span>
<span class="special">{</span>
    <span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
    <span class="keyword">int</span> <span class="identifier">y</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">point</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">x</span><span class="special">(</span><span class="number">0</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="number">0</span><span class="special">)</span> <span class="special">{}</span>
    <span class="identifier">point</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">x</span><span class="special">(</span><span class="identifier">x</span><span class="special">),</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> <span class="special">{}</span>

    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">other</span><span class="special">)</span> <span class="keyword">const</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">x</span> <span class="special">==</span> <span class="identifier">other</span><span class="special">.</span><span class="identifier">x</span> <span class="special">&amp;&amp;</span> <span class="identifier">y</span> <span class="special">==</span> <span class="identifier">other</span><span class="special">.</span><span class="identifier">y</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
      and you wish to use it as the key for an <code class="computeroutput"><span class="identifier">unordered_map</span></code>.
      You need to customise the hash for this structure. To do this we need to combine
      the hash values for <code class="computeroutput"><span class="identifier">x</span></code> and
      <code class="computeroutput"><span class="identifier">y</span></code>. The function <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code> is supplied for
      this purpose:
    </p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">point</span>
<span class="special">{</span>
    <span class="special">...</span>

    <span class="keyword">friend</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">point</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">p</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">seed</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
        <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">x</span><span class="special">);</span>
        <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code><span class="special">(</span><span class="identifier">seed</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">y</span><span class="special">);</span>

        <span class="keyword">return</span> <span class="identifier">seed</span><span class="special">;</span>
    <span class="special">}</span>

    <span class="special">...</span>
<span class="special">};</span>
</pre>
<p>
      Calls to hash_combine incrementally build the hash from the different members
      of point, it can be repeatedly called for any number of elements. It calls
      <code class="computeroutput"><a class="link" href="reference.html#boost.hash_value_idp39522320">hash_value</a></code> on the supplied
      element, and combines it with the seed.
    </p>
<p>
      Full code for this example is at <a href="../../../libs/functional/hash/examples/point.cpp" target="_top">/libs/functional/hash/examples/point.cpp</a>.
    </p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top">
<p>
        When using <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code>
        the order of the calls matters. </p>
<pre class="programlisting">
    std::size_t seed = 0;
    boost::hash_combine(seed, 1);
    boost::hash_combine(seed, 2);
</pre>
<p>
results in a different seed to:
</p>
<pre class="programlisting">
    std::size_t seed = 0;
    boost::hash_combine(seed, 2);
    boost::hash_combine(seed, 1);
</pre>
<p>
If you are calculating a hash value for data
        where the order of the data doesn't matter in comparisons (e.g. a set) you
        will have to ensure that the data is always supplied in the same order.
      </p>
</td></tr>
</table></div>
<p>
      To calculate the hash of an iterator range you can use <code class="computeroutput"><a class="link" href="reference.html#boost.hash_range">boost::hash_range</a></code>:
    </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">some_strings</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="reference.html#boost.hash_range">boost::hash_range</a></code><span class="special">(</span><span class="identifier">some_strings</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">some_strings</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
</pre>
<p>
      Note that when writing template classes, you might not want to include the
      main hash header as it's quite an expensive include that brings in a lot of
      other headers, so instead you can include the <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">functional</span><span class="special">/</span><span class="identifier">hash_fwd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code>
      header which forward declares <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>,
      <code class="computeroutput"><a class="link" href="reference.html#boost.hash_range">boost::hash_range</a></code> and
      <code class="computeroutput"><a class="link" href="reference.html#boost.hash_combine">boost::hash_combine</a></code>.
      You'll need to include the main header before instantiating <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>.
      When using a container that uses <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code>
      it should do that for you, so your type will work fine with the boost hash
      containers. There's an example of this in <a href="../../../libs/functional/hash/examples/template.hpp" target="_top">template.hpp</a>
      and <a href="../../../libs/functional/hash/examples/template.cpp" target="_top">template.cpp</a>.
    </p>
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      James<p>
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        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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